Estimating values of Fe3+/ΣFe in pyroxenes with the electron microprobe

Values of Fe³⁺/ ΣFe (ΣFe = Fe³⁺ + Fe²⁺) were determined for six (6) pyroxenes, (five (5) diopsides and one (1) enstatite), using the single dissolution technique of Fritz and Popp, 1985 (FP85). These same pyroxenes were analyzed with the electron microprobe (EMP), and values of Fe³⁺/ΣFe were determined using charge balance from average EMP compositions. Typically, these averages were based on thirty (30) analyses on multiple grains of the same sample, although some pyroxenes were characterized with as few as 9 and as many as 120 analyses.

Differences between average values of Fe³⁺/ΣFe determined using the two methods were ≤ 0.02 for pyroxenes with Fe-contents >9 wt% FeO_T (all Fe as FeO_T). Pyroxenes with lower Fe contents of approximately 4 to 6 wt% FeO_T exhibit differences that fall between 0.02 and 0.06. One pyroxene, with the lowest Fe-content (1.79 wt% FeO_T; 0.055 atoms per formula unit or apfu), exhibits the largest difference Fe³⁺/ ΣFe difference between EMP and FP85 of 0.14. Values of the standard deviation (1σ), based on average values of Fe³⁺/ ΣFe (EMP), range from 0.03 to 0.22, which are also inversely correlated to the Fe content. For pyroxenes with FeO_T > 4.0 wt%, and with compositions similar to those examined in this study, it is possible to use charge balance to determine values of Fe³⁺/ΣFe(EMP) to better than ±0.07.